This invention relates to marine seismic prospecting and, more particularly, to a stabilized connection system for attaching equipment to seismic streamers.
A marine seismic streamer is a cable, typically several thousand meters long, that contains arrays of hydrophones and associated electronic equipment along its length. One purpose of the streamer is to position the hydrophone array at a known depth and position relative to a towing vessel in a survey area. Externally mounted equipment, such as depth controllers, called “birds,” lateral-position controllers, emergency recovery pods, and acoustic pods, performs the functions of positioning, controlling, and recovering the cable.
Individual external devices are attached to the streamer at various positions along its length. In one conventional streamer connection system, a collar arrangement uses a hinge and latch mechanism for operation. Examples of this kind of system are described in U.S. Pat. No. 5,507,243, “Connector For Underwater Cables,” Apr. 16, 1996, to Oneil J. Williams et al. and in U.S. Pat. No. 5,709,497, “Latching Device,” Jan. 20, 1998, to David W. Zoch et al. Another conventional connection system uses C-shaped collars that slip radially onto the cable and slide axially into position. Examples of this system are described in U.S. Pat. No. 6,263,824, “Connection System for Connecting Equipment to Underwater Cables,” Jul. 24, 2001, to André W. Olivier. The C-shaped collars dispense with the need for hinges and latches. Yet another connection system using C-shaped collars specially designed to maintain themselves coaxially aligned even when the cable is subjected to severe bending forces is described in commonly owned pending U.S. patent application Ser. No. 11/669,650, “Streamer Connection System.” The disclosures of all the references mentioned in this paragraph are incorporated by reference. All these kinds of outer collars, with external devices attached, are attached around inner collars affixed to the cable as the cable is payed out from the back deck of a survey vessel. Circular cylindrical inner surfaces on the outer collars ride on races formed on the exterior of the inner collars. This allows the streamer to rotate inside the outer collars without also rotating the external devices.
Most external devices attach to a pair of outer collars separated by a standard distance of about 57 cm (22.5 in). This separation is set by the fixed spacing of spacer blocks and communication coil mounts within the streamer. These relatively rigid objects provide a firm structure under the skin of the streamer about which the inner collars can be clamped. Because most early streamers were constructed with the standard separation of about 57 cm, external devices were constructed with fixed attachment points 57 cm apart. Some newer streamers, however, have been constructed with communication coil mounts and spacer blocks at different intervals not compatible with the 57-cm fixed spacing of conventional external devices.
Thus, there is a need for a streamer connection system that can adapt to streamers having communication coils mounts and spacer blocks separated by distances not equal to the fixed spacing of attachment points on external devices.